The invention relates to a filtering apparatus which is adapted for the filtration of a plastic melt.
Filtering apparatuses of the described type are disclosed in EP 0 247 468 A2, as well as in the treatise "Verbesserte Extrusion technologie durch Gro.beta.fl achen-Schmelzfiltration," Kunststoffe 70 (1980) 11, pp. 753-758. Such filtering apparatuses are typically constructed as large-surface filters employing hollow cylindrical filter elements, which are closed at one end and connected to the filter housing at the other end. The filtering apparatuses are commonly used in the processing of plastic melts, in particular in the production of yarns and films, as well as in the production of thin-walled extruded parts, so as to guarantee high quality products and an economical service life of molds and/or extrusion dies.
With the use of such filtering apparatuses, it is desirable to have identical residence times for the melts flowing through the filter, since the properties of the plastic melt change under the action of temperature and residence time. In the extreme case, these factors will lead to the decomposition of the plastic melt.
As is described in EP 0 247 468 A2, the filtering apparatuses may be hydraulically designed, so that no dead zones develop, if possible, in which the flow stagnates, and so that the melt obtains the same residence time over the entire filter surface. To this end, EP 0 247 468 A2 discloses that by means of the installation of displacers, it is possible to shorten the residence time and keep the residence time distribution within narrow limits. It is further known that the improvement of the melt feed through conical bores or contoured housings leads to a more favorable distribution of the residence time.
In practice, it has been found that with the use of tubular filter elements, which are commonly referred to as filter cartridges, it is possible to obtain approximately identical residence times over the entire filter surface. However, zones with longer residence times develop at the ends of the peripheral filter surface, since there the filter medium, such as wire mesh, metallic fiber web, etc., is compressed, so as to be connected at its one end to a metal ring, the so-called adapter, and at its other end to a metal cap.
In particular, filter cartridges consisting of a folded wire mesh, present edge zones with a higher flow resistance. Unduly long residence times occur primarily at the outlet end of the filter chamber.
Accordingly, it is the object of the invention to substantially eliminate dead zones, or even long residence times of the melt at the outlet ends of the filter chambers, where the filter cartridge is secured by means of an adapter to the end surface of the filter chamber.